Many modern day cements are composed of separate components which, when thoroughly mixed together, undergo a chemical reaction resulting in a substance which produces a very strong and effective bond between dissimilar objects. Various epoxy and acrylic cements fall into this category. In some cases, both components of the cement are liquids; in other cases, only one of the components is a liquid, the other being a powdered solid material.
While the present invention can be used to prepare and apply a variety of such cements, it is especially useful in the preparation and application of so-called bone cement used to anchor and support artificial joint components and other prostheses in natural bone. Accordingly, it will be described here specifically in that context.
The currently preferred bone cement is polymethylmethacrylate or so-calledPMMA. PMMA is comprised of a powdered polymer and a liquid monomer catalyst. Upon mixing, these components polymerize within minutes so as to form a firm rigid bond between the prosthesis and the surrounding bone structure in which the prosthesis is placed.
The present procedure for preparing and applying PMMA bone cement is to remove the powdered catalyst and liquid catalyst from their separate containers and pour them into a mixing bowl, one after the other. Then the two cement components are mixed together by stirring them with an inert spatula. Usually, the mixing bowl is fitted with a vacuum device and a filterf to minimize the discharge of monomer vapors from the bowl. These vapors are toxic and could cause discomfort and injury to operating room personnel. The stirring processcontinues until the two components partially polymerize forming a substance having the consistency of putty or dough. That partially cured cement is then pressed into the bone structure in which the prosthesis is to be placed so as to form a bed for receiving the prosthesis.
For example, in a total hip replacement, a bed of cement is pressed into the patient's acetabulum by hand so as to form a bed for the acetabular component of the hip prosthesis. That component is a cup-like object which is usually made of plastic and defines a socket for receiving the femural component of the hip prosthesis. The latter is basically a ball formed at the end of a long stem. That stem is inserted into the patient's femural medullary canal after removal of the femural neck by known procedures. Prior to such insertion, the medullary canal is reamed out and packed with bone cement. Usually, the cement taken from the bowl is extruded into that canal under pressure using a cement gun or syringe which may be likened to a caulking gun. Then the stem of the femural prosthesis is inserted into the femural canal and positioned so that its ball is properly received in the acetabular component so as to allow a substantially full range of flexure of the new hip joint.
Not infrequently, one or both of the hip prostheses loosens, requiring reoperation to correct the failure. Sometimes, such loosening of the hip prosthesis has drastic consequences such as protrusion of the acetabulum or proximal femur fracture. In many cases, such failures have been traced to problems with the bone cement bond between the prosthesis and the bone structure.
More particularly, porosities or voids induced in the cement by the now-practiced mixing techniques cause a drastic reduction in its fatigue strength. Consequent cracking and fragmentation of the cement results in component loosening, almost invariably followed by various medical complications requiring a second surgical procedure. This porosity problem is widely recognized since, when mixing together the cement components in the mixing bowl, surgeons are cautioned not to whip or beat the mixture as that tends to produce pockets and voids which weaken the cement. However, even with careful mixing following normal procedures, a substantial number of pores and voids still remain in the cement body.
Also, if the cement body is to have a uniform strength throughout its extent, it is essential that the cement components be brought togethr as a homogeneous mixture so that the components throughout the mixture are in proper proportion. This is very difficult to do in a mixing bowl. Invariably, the degree of quality of the mixing varies according to the stirring motion used by the individual doing the mixing. A particular individual may stir for the requisite time to produce a partially polymerized cement mixture which overall has the proper consistency for placement in the body. However, small regions within that body may not polymerize completely because of inadequate mixing of the cement components in those regions. Those regions not only constitute weaknesses in the resultant cement bond, they also cause leakage of the unbound catalyst monomer, which is a toxic substance, into the surrounding bone structure. In fact, such leakage has already been identified as a major source of PMMA toxicity in such patients.
Still further, some problems with the bone cement bond can be attributed to the mode of applying the cement mixture to the bone structure prior to seating the prosthesis. As noted previously, the generally accepted technique for bedding the cement mixture is to press the dough-like substance into the bone structure or to extrude or inject it under static pressure using a cement syringe or gun. Even with the cement mixture well contrained in a bone cavity, which in practice is very hard to achieve, the influence of such statically applied pressure on cement penetration into bone is very limited. That influence cannot be increased by simply increasing the applied pressure without damaging the surrounding bone structure.
Finally, some of the prior apparatus used for this purpose are rather large and bulky and comprise several components, some of which are too expensive to be deemed disposable. Yet the removal of the polymerized cement from those parts after each use is time consuming and therefore also expensive.